1. Field of the Invention
The present invention relates to an optical information recording/reproducing method and an optical information recording/reproducing apparatus for recording data on an erasable type optical disk by a mark edge recording method, and more particularly to recording compensation for accurately controlling an edge position of a recording mark.
2. Description of the Related Art
The development of an optical information recording medium which can record and reproduce such information signals as video and audio signals, particularly optical disks, is active. One optical disk media which can record information at high density is a phase change type optical disk. Data is recorded on a phase change type optical disk by emitting a laser beam narrowed down to a 1 xcexcm or less diameter on a rotating disk, so as to heat and fuse the recording film. Depending on the strength of the recording light beam, the temperature on the recording film when the beam reaches the disk and the cooling process differ, and a phase change of the recording film occurs between the crystal state and amorphous state.
When the light beam is strong (called xe2x80x9cpeak power levelxe2x80x9d), the recording film becomes amorphous since the recording film is heated beyond the fusing point, fuses, then rapidly cools down. When the light beam is at medium strength (called xe2x80x9cbias power levelxe2x80x9d), the recording film is crystallized since the recording film is maintained at a temperature higher than the crystallization temperature but is lower than the fusing point. The amorphous part is called a xe2x80x9cmarkxe2x80x9d, and the crystallized part is called a xe2x80x9cspacexe2x80x9d. The method of recording data by assigning information to the length of the mark and space is called the xe2x80x9cmark edge recording methodxe2x80x9d. Since a phase change optical disk can create marks by fusing the recording film at a peak power level, whether the recording film is in an amorphous state or crystal state, simultaneously erasing old data and recording new data using one light beam, that is, direct overwriting, is possible.
However, when a long mark is recorded by mark edge recording, if a light beam at peak power level is emitted to the mark part at a predetermined intensity, the width of the mark becomes gradually wider toward the end of the mark, since heat accumulates on the recording film. This causes signal quality deterioration, such as incomplete erasure, during direct overwriting. To prevent this, recording marks by a light beam which switches alternately between peak power level and bias power level at high-speed between mark blocks, that is, multi-pulse recording, is effective. By this method, the heat accumulating effect at the latter half of a mark subsides, and a mark with a predetermined width from start to end can be created.
For reproducing, a light beam which is weak enough not to cause a phase change of the recording film is emitted, and the intensity of the reflected light is detected by a photo-detector. Since the reflectance of the mark part, which is amorphous, can be significantly different from the reflectance of the space part, which is crystallized, by choosing the material of the recording film and the configuration of the recording film and the protective layer, reproducing signals of the recorded data can be obtained by detecting the difference of the reflected light intensity between the mark part and the space part.
A possible way to improve the recording density of the phase change type optical disk is to decrease the length of the mark and the space to be recorded. If a space length is short, however, the heat at the end edge of the recorded mark conducts through the space part and affects the temperature rising at the start edge of the next mark, or the heat at the start edge of the mark recorded next affects the cooling process at the end edge of the previous mark, that is, thermal interference occurs. As a result, the edge position of the mark changes, and the data error rate at reproduction increases. This phenomena will be described with reference to FIGS. 9(a) to (e). FIG. 9(a) is a waveform diagram where the recording data is shown in binary, (b) is a waveform diagram indicating the intensity of laser beam emission in binary, where one mark corresponds to a plurality of short pulses, as mentioned above. (c) is an illustration of recording marks created on the disk, (d) is a waveform diagram of the reproducing signals of the recording mark in (c), and (e) is a waveform diagram when the reproducing signals are binarized. In FIGS. 9(a) to (e), a case where a short space is before and after a long mark is shown as a characteristic example of the influence of thermal interference. If the start and end edges of the emission pulses in (b) are placed at the same positional relationship as the start and end edges of the recording data in (a), the start and end edges of the recording mark extend as shown in (c), regardless of the length of spaces before and after. In FIGS. 9(a) to (e), the amount of shift at the front edge and rear edge of the mark at the center are denoted by Es1 and Es2. As a result, the mark block becomes longer than the desired length, as shown in FIG. 9(d), and an edge shift occurs when the waveform is binarized, as shown in FIG. 9(e).
The amount of fluctuation of the edge position due to the thermal interference differs depending on the length of the space before and after the target mark. Therefore, to solve this problem, Japanese Patent Application Laid-Open No. S63-48617 discloses a technology to compensate for the fluctuation of the edge position due to thermal interference by changing the start edge position of the recording pulse of the mark part in advance according to the length of the space before the mark. The amount of change of the start edge position is determined by recording a test pattern comprised of a plurality of combinations of marks and spaces of different lengths in advance, reproducing the test pattern, and measuring the deviation between the edge positions of the reproducing signals and the target values. FIG. 10 shows an example of a test pattern. The pattern of the test pattern is different depending on the data modulation system, but includes at least the shortest mark and space and the longest mark and space which become the references for measuring edge positions. If it is assumed that the channel bit length to be the unit of length of marks and spaces is T, then the shortest channel bit length of recording data is 3T, and the longest is 11T in the case of 8-16 modulation. For example, the edge interval from the reference mark (11T) in the reproducing signal is different between a case where the length of the subsequent space of the 3T target mark is 3T and a case where the length is 11T. A method of compensating for the edge shift with a deviation amount determined by recording such test pattern will be described with reference to FIGS. 11(a) to (e). FIGS. 11(a) to (e) show the recording data, laser emission signal, recording marks, reproducing signal and binary signal respectively, just like FIG. 10. Edge compensation is executed by shifting the front edge pulse and rear edge pulse of the laser emission signal by Es1xe2x80x2 and Es2xe2x80x2 corresponding to the above mentioned shift amounts Es1 and Es2, as shown in FIG. 11(b). In this way, a desired length of marks can be recorded as shown in FIGS. 11(c) to (e), which can prevent data reproducing errors.
With such a conventional optical information recording/reproducing method and optical information recording/reproducing apparatus, however, the fluctuation of the edge position of reproducing data can be decreased when reproducing data with the same apparatus as the apparatus which recorded the data, but if the data is reproduced with an apparatus which is different from the apparatus which recorded the data, the fluctuation of the edge position increases, influenced by the laser spot shapes of recording and reproducing, the characteristic irregularity of the recording film, and the characteristic irregularity of the reproducing system, that is, reproducing compatibility cannot be implemented. This problem will be described below.
The correction amount of the edge position generated by thermal interference is determined by the recording/reproducing of the test pattern, but the correct amount is influenced by the group-delay characteristics of the reproducing optical system and reproducing circuit. This is because the test pattern includes short marks and spaces and long marks and spaces, as mentioned above. In order to learn the correction amount of an edge, the interval between the front or rear edge of the long mark to be a reference and the edge of the short mark, which is the measurement target, must be measured. If the reproducing system has a group-delay characteristic, the reproducing signals of a short mark and space delay or advance from the reproducing signals of a long mark and space, so the shift amount of the edge of the marks cannot be correctly measured. Therefore, even if an apparatus having the group-delay characteristic learns the correction amount of the edge and records such that the shift amount in the binary signals is 0, an edge shift occurs to the reproducing signals if the data is reproduced by an apparatus for which the group-delay characteristic is different, or which does not exist at all. In other words, the lengths of the same mark and space differ after binarization depending on the reproducing apparatus.
With the foregoing in view, it is an object of the present invention to provide an optical information recording/reproducing method which accurately compensates for the edge shift amount caused by the thermal interference between recording marks and easily implements compatibility among reproducing apparatuses.
To achieve the above object, an optical information recording/reproducing method in accordance with the present invention is an optical information recording/reproducing method where length information on a mark block and a space block is used as data, the data is recorded in the form of changes of a local optical constant on a recording layer by emitting a light beam to an optical medium while switching the intensity of the light beam according to one or more recording pulses for the mark block, and the data is reproducing by detecting the changes of the above mentioned local optical constant by a light beam with a predetermined intensity. The optical information recording/reproducing method comprises a reproducing learning step for switching the frequency characteristic of a reproducing system for detecting the change of the above mentioned local optical constant to a predetermined frequency characteristic, and a recording learning step for reproducing a test pattern recorded on the above mentioned optical information recording medium and correcting the start edge or the end edge position of the above mentioned recording pulse for each combination of length of the above mark block and length of the preceding or subsequent space block, so that the edge position of the binarized reproducing data comes to the desired position.